1. Field of the Invention
The present invention generally relates to transverse flux induction heating and more particularly to transverse flux induction heating with induction coil turns having an adjustable coil pitch.
2. Description of Related Art
A conventional transverse flux induction apparatus 100 is shown in exploded view in FIG. 1. The apparatus includes a coil pair comprising a first and second coil, 112 and 114, respectively, configured as two-turn coils. Transverse (substantially perpendicular to the longitudinal direction of workpiece 120, as indicated by the arrow labeled xe2x80x9cXxe2x80x9d) segments and longitudinal (approximately parallel with the longitudinal direction of workpiece 120) segments of each coil form a generally rigid and continuous coil. The pole pitch, xcfx84, is fixed for each turn of the two-turn first and second coil segments. A magnetic flux concentrator 116, shown as laminated steel plates, surrounds the first and second coils generally in all directions except for coil surfaces that face workpiece 120, which is a continuous metal workpiece (such as a metal strip) that will be inductively heated as it passes between the coil pair. For clarity of coil arrangements in FIG. 1, the concentrator for coil 112 is shown in broken view and the concentrator for coil 114 is not shown. In this exploded view, coil gap, gc, is exaggerated. In typical applications, the coil gap is generally only larger than the thickness, ds, of the workpiece as to allow unobstructed travel of the strip between the coils. When in-phase ac electric power is applied to the terminals of the first and second coil sections (that is, for example, instantaneously positive power to terminals 1 and 3, and instantaneously negative power to terminals 2 and 4), the current flowing through the first and second coils establish a common magnetic flux that passes perpendicularly through the workpiece as illustrated by the exemplary dashed flux line in FIG. 1, with the arrows indicating the direction of the flux.
FIG. 2 is a graph plotting the temperature across the transverse of a workpiece. Transverse points on the workpiece (x-axis) are normalized with 0.0 representing the center of the transverse and +1 and xe2x88x921 representing the opposing edges of the transverse. Curve 81 in FIG. 2 is a plot of the typical cross sectional temperature distribution for a workpiece that is inductively heated by the common magnetic flux established in a conventional transverse flux coil pair. If the workpiece enters the transverse flux induction apparatus 100 with its edges at temperatures lower than the temperature at the center of the workpiece, this effect could be used to an advantage to more evenly heat the workpiece across its width or transverse. However, if the workpiece enters the apparatus with a uniform temperature across its transverse, the edges will be overheated. For this condition, it would be ideal to inductively heat the workpiece uniformly across its transverse, as indicated by line 82 in FIG. 2. The frequency of the power source can be varied to some extent to compensate for the edge overheating effect, at the expense of a significant increase in the cost of the power supply. Alternatively, discrete edge heaters, in addition to a main induction heating apparatus, can be used to compensate for this non-uniform cross sectional heating. See, for example, U.S. Pat. No. 5,156,683 entitled Apparatus for Magnetic Induction Edge Heaters with Frequency Modulation. However, this approach requires additional equipment and a more complex control system.
Therefore, there exists the need for a transverse flux induction heating apparatus and method that will provide a quick and efficient method of reconfiguring the coil pair to provide a variable degree of heating across the cross section of a workpiece, including selective edge heating, without changing the frequency of the induction power source or adding separate edge heaters.
In one aspect the present invention is a transverse flux induction heating apparatus and method that allows continuous adjustment of the operating pole pitch for a coil pair used in the apparatus to heat the transverse of the workpiece to a substantially uniform temperature. These and other aspects of the invention are set forth in the specification and claims.